If I were gonna bet on it.. the first explanation I would gravitate towards would be that these teens might have been the children of single teenage mothers/fathers themselves.. and lived through the hardship of that situation first hand.
Also games and social networks - they drive teens crazy and tempt them to lock themselves in their homes for much of their free time. When you stay at home, you get less sex.
On the whole, teens are replacing sex with more computer time.